Can i use oil of oregano in addition to augmentin (amoxicillin and clavulanate) (i have a sinus infection and read about the benefits of this natural remedy). ?

YES: There are no known interactions between oregano oil & augmentin (amoxicillin and clavulanate). Augmentin (amoxicillin and clavulanate) is a good broad-spectrum antibiotic but many sinus infections are viral &/or fungal. Oregano oil kills viruses, fungi & bacteria so is reasonable to take as well. Also consider pelargonium sidoides (sold as umcka), an herb proven effective for sinus infections. I also advise taking a probiotic for anyone on antibioitics.